April 2010 - Energy Harvester Produces Power from Local Environment, Eliminating Batteries in Wireless Sensors

April 2010
I N
T H I S
I S S U E
our new look 2
dual output step-down
regulator with DCR sensing
in a 5mm × 5mm QFN 9
accurate battery gas
Volume 20 Number 1
Energy Harvester Produces Power
from Local Environment, Eliminating
Batteries in Wireless Sensors
Michael Whitaker
gauges with I2C interface 12
dual buck regulator
operates outside of
AM radio band 20
eight 16-bit VOUT DACs in a
4mm × 5mm QFN 24
Advances in low power technology are making it easier to create
wireless sensor networks in a wide range of applications, from
remote sensing to HVAC monitoring, asset tracking and industrial
automation. The problem is that even wireless sensors require
batteries that must be regularly replaced—a costly and cumbersome
maintenance project. A better wireless power solution would be to
harvest ambient mechanical, thermal or electromagnetic energy from
the sensor’s local environment.
dual-phase converter for
1.2V at 50A with DCR
sensing 28
Typically, harvestable ambient power is on the order of tens of microwatts, so energy harvesting
requires careful power management in order to successfully capture microwatts of ambient power
and store it in a useable energy reservoir. One common form of ambient energy is mechanical
vibration energy, which can be caused by motors running in a factory,
airflow across a fan blade or even by a moving vehicle. A piezoelectric transducer can be used to convert these forms of vibration energy
into electrical energy, which in turn can be used to power circuitry.
To manage the energy harvesting and the energy release to the system,
the LTC®3588-1 piezoelectric energy harvesting power supply (Figure 1)
integrates a low loss internal bridge rectifier with a synchronous stepdown DC/DC converter. It uses an efficient energy harvesting algorithm
to collect and store energy from high impedance piezoelectric elements,
which can have short-circuit currents on the order of tens of microamps.
Piezoelectric energy harvesting power supply
Energy harvesting systems must often support peak load currents
that are much higher than a piezoelectric element can produce, so
the LTC3588-1 accumulates energy that can be released to the load in
short power bursts. Of course, for continuous operation these power
(continued on page 4)
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The LTC3588-1 interfaces with the piezo through its internal
low loss bridge rectifier accessible via the PZ1 and PZ2
pins. The rectified output is stored on the VIN capacitor.
At typical 10µA piezoelectric currents, the voltage drop
associated with the bridge rectifier is on the order of 400mV.
(continued from page 1)
bursts must occur at a low duty cycle,
such that the total output energy during
the burst does not exceed the average
source power integrated over an energy
accumulation cycle. A sensor system
that makes a measurement at regular
intervals, transmits data and powers
down in between is a prime candidate
for an energy harvesting solution.
KEY TO HARVESTING IS
LOW QUIESCENT CURRENT
The energy harvesting process relies on a
low quiescent current energy accumulation
phase. The LTC3588-1 enables this through
an undervoltage lockout (UVLO) mode with
a wide hysteresis window that draws less
than a microamp of quiescent current. The
UVLO mode allows charge to build up on
an input capacitor until an internal buck
converter can efficiently transfer a portion of the stored charge to the output.
Figure 2 shows a profile of the quiescent
current in UVLO, which is monotonic with
VIN so that a current source as low as
700nA could charge the input capacitor
1µF
6V
CSTORAGE
25V
to the UVLO rising threshold and result
in a regulated output. Once in regulation, the LTC3588-1 enters a sleep state in
which both input and output quiescent
currents are minimal. For instance, at
VIN = 4.5V, with the output in regulation, the quiescent current is only 950nA.
The buck converter then turns on and
off as needed to maintain regulation.
Low quiescent current in both the sleep
and UVLO modes allows as much energy
to be accumulated in the input reservoir capacitor as possible, even if the
source current available is very low.
800
700
SW
VOLTAGE (V)
–40°C
500
400
300
PGOOD
D0, D1
0
0
2
3
VIN (V)
4
5
Figure 2. IVIN in UVLO vs VIN
4 | April 2010 : LT Journal of Analog Innovation
6
85°C
1600
25°C
1400
1200
–40°C
1000
6
2
D1 = D0 = 0
2000
8
800
VOUT
600
PGOOD = LOGIC 1
0
200
400
TIME (s)
OUTPUT
VOLTAGE
SELECT
When VIN reaches the UVLO rising threshold, the high efficiency integrated synchronous buck converter turns on and
begins to transfer energy from the input
capacitor to the output capacitor. The
buck regulator uses a hysteretic voltage algorithm to control the output via
internal feedback from the VOUT sense pin.
It charges the output capacitor through
an inductor to a value slightly higher
than the regulation point by ramping the
inductor current up to 250mA through
an internal PMOS switch and then ramping it down to zero current through an
internal NMOS switch. This efficiently
delivers energy to the output capacitor.
2200
10
4
2
GND
2400
VIN
12
100
VOUT
47µF
6V
1800
14
200
10µH
VOUT
VIN2
16
600
LTC3588-1
CAP
CSTORAGE = 22µF, COUT = 47µF
20 NO LOAD, IVIN = 2µA
18
25°C
1
PZ2
VIN
22
85°C
0
PZ1
4.7µF
6V
D1 = D0 = 1
900
IVIN (nA)
ADVANCED CERAMETRICS PFCB-W14
IVIN (nA)
1000
Figure 1. Piezoelectric energy
harvesting power supply
600
Figure 3. A 3.3V regulator start-up profile
400
2
4
6
8
10 12
VIN (V)
14
16
Figure 4. IVIN in sleep vs VIN
18
design features
Figure 8. Block diagram of the LTC3588-1
VIN
20V
INTERNAL RAIL
GENERATION
CAP
PZ1
SW
VIN2
PZ2
If the input voltage falls below the
UVLO falling threshold before the output voltage reaches regulation, the buck
converter shuts off and is not turned on
again until the input voltage rises above
the UVLO rising threshold. During this time
the leakage on the VOUT sense pin is no
greater than 90nA and the output voltage remains near the level it had reached
when the buck was switching. Figure 3
shows a typical start-up waveform of the
LTC3588-1 charged by a 2µA current source.
When the synchronous buck brings the
output voltage into regulation the converter enters a low quiescent current sleep
state that monitors the output voltage with
a sleep comparator. During this operating
mode, load current is provided by the buck
output capacitor. When the output voltage
falls below the regulation point the buck
regulator wakes up and the cycle repeats.
This hysteretic method of providing a regulated output minimizes losses associated
with FET switching and makes it possible
to efficiently regulate at very light loads.
EFFICIENCY (%)
50
40
VOUT = 3.6V
VOUT = 3.3V
VOUT = 2.5V
VOUT = 1.8V
20
10
1µ
10µ
100µ
1m
10m
LOAD CURRENT (A)
Figure 5. Efficiency vs ILOAD
100m
PGOOD
LTC3588-1
The buck delivers up to 100mA of average load current when it is switching.
Four output voltages, 1.8V, 2.5V, 3.3V and
3.6V, are pin selectable and accommodate
powering of microprocessors, sensors and
wireless transmitters. Figure 4 shows the
extremely low quiescent current while in
regulation and in sleep, which allows for
efficient operation at light loads. Although
the quiescent current of the buck regulator while switching is much greater than
the sleep quiescent current, it is still a
small percentage of the load current,
which results in high efficiency over a
wide range of load conditions (Figure 5).
The buck operates only when sufficient
energy has been accumulated in the input
capacitor, and it transfers energy to the
output in short bursts, much shorter than
the time it takes to accumulate energy.
When the buck operating quiescent current is averaged over an entire accumulation/burst period, the average quiescent
current is very low, easily accommodating sources that harvest small amounts
of ambient energy. The extremely low
quiescent current in regulation also allows
the LTC3588-1 to achieve high efficiency at
loads under 100µA as shown in Figure 5.
20
18
60
30
PGOOD
COMPARATOR
12
70
VOUT
2
D1, D0
VIN = 5V
80
0
BANDGAP
REFERENCE
VIN = 18V, LEAKAGE AT PZ1 OR PZ2
16
9
BRIDGE LEAKAGE (nA)
90
GND
SLEEP
RECTIFIED PIEZO VOLTAGE (V)
100
BUCK
CONTROL
UVLO
INCREASING
VIBRATION ENERGY
6
3
14
12
10
8
6
4
2
0
0
10
20
PIEZO CURRENT (µA)
30
Figure 6. Typical piezoelectric load lines for Piezo
Systems T220-A4-503X
0
–55
–10
35
80
125
TEMPERATURE (°C)
170
Figure 7. The internal bridge rectifier outperforms
discrete solutions
April 2010 : LT Journal of Analog Innovation | 5
Though an energy harvesting system can eliminate the need
for batteries, it can also supplement a battery solution.
Figure 9. Piezo energy
harvester with battery
backup
Ambient vibrations can be characterized
in order to select a piezo with optimal
characteristics. The frequency and force
of the vibration as well as the desired
interval between use of the LTC3588-1’s
output capacitor reservoir and the amount
of energy required at each burst can
help to determine the best piezoelectric
element. In this way, a system can be
designed so that it performs its task as
often as the amount of available energy
allows. In some cases, optimization of the
piezoelectric element may not be necessary as just the capability to harvest any
amount of energy may be attractive.
MIDE V21BL
IR05H40CSPTR
1µF
6V
9V
BATTERY
100µF
16V
PZ2
VIN
PGOOD
CAP
LTC3588-1
4.7µF
6V
Piezoelectric elements convert mechanical energy, typically vibration energy, into
electrical energy. Piezoelectric elements
can be made out of PZT (lead zirconate
titanate) ceramics, PVDF (polyvinylidene
fluoride) or other composites. Ceramic
piezoelectric elements exhibit a piezoelectric effect when the crystal structure of
the ceramic is compressed and internal
dipole movement produces a voltage.
Polymer elements comprised of long-chain
molecules produce a voltage when flexed
as molecules repel each other. Ceramics
are often used under direct pressure while
a polymer can be flexed more readily.
A wide range of piezoelectric elements are
available and produce a variety of open
circuit voltages and short-circuit currents.
The open circuit voltage and short-circuit
current form a “load line” for the piezoelectric element that increases with available vibration energy as shown in Figure 6.
The LTC3588-1 can handle up to 20V at its
input, at which point a protective shunt
safeguards against an overvoltage condition on VIN. If ample ambient vibration
causes a piezoelectric element to produce
D1
D0
PGOOD
10µH
SW
VOUT
VIN2
REAPING VIBRATION ENERGY
6 | April 2010 : LT Journal of Analog Innovation
PZ1
PZ1
GND
VOUT
3.3V
47µF
6V
PZ2
more energy than the LTC3588-1 needs, the
shunt consumes the excess power, effectively clamping the piezo on its load line.
The LTC3588-1 interfaces with the piezo
through its internal low loss bridge rectifier accessible via the PZ1 and PZ2 pins.
The rectified output is stored on the
VIN capacitor. At typical 10µA piezoelectric currents, the voltage drop associated
with the bridge rectifier is on the order
of 400mV. The bridge rectifier also suits a
variety of other input sources by featuring less than 1nA of reverse leakage at
125°C (Figure 7), a bandwidth greater
than 1MHz and ability to carry 50mA.
Figure 10. Electric field
energy harvester
COPPER PANEL
(12in × 24in)
1µF
6V
10µF
25V
OPTIONS FOR ENERGY STORAGE
Harvested energy can be stored on the
input capacitor or the output capacitor.
The wide input range takes advantage of
the fact that energy storage on a capacitor is proportional to the square of the
capacitor voltage. After the output voltage
is brought into regulation any excess
energy is stored on the input capacitor
and its voltage increases. When a load
exists at the output, the buck can efficiently transfer energy stored at a high
voltage to the regulated output. While
PANELS ARE PLACED 6"
FROM 2' × 4' FLUORESCENT
LIGHT FIXTURES
PZ1
PZ2
VIN
PGOOD
CAP
LTC3588-1
4.7µF
6V
D1
D0
GND
PGOOD
10µH
SW
VOUT
VIN2
COPPER PANEL
(12in × 24in)
3.3V
10µF
6V
design features
The LTC3588-1 can harvest other sources of energy
besides the ambient vibration energy available from a
piezoelectric element. The integrated bridge rectifier allows
many other AC sources to power the LTC3588-1.
Figure 11. AC line powered
3.6V buck regulator
DANGER! HIGH VOLTAGE!
150k
150k
120VAC
60Hz 150k
150k
1µF
6V
10µF
25V
DANGEROUS AND LETHAL POTENTIALS ARE PRESENT IN OFFLINE CIRCUITS!
PZ1
PZ2
VIN
PGOOD
CAP
LTC3588-1
PGOOD
10µH
VOUT
3.6V
SW
VOUT
VIN2
4.7µF
6V
Before proceeding any further, the reader is warned that caution must be used in
the construction, testing and use of offline circuits. Extreme caution must be
used in working with and making connections to these circuits.
100µF
6V
D1
D0
REPEAT: Offline circuits contain dangerous, AC line-connected high voltage
potentials. Use caution. All testing performed on an offline circuit must be done
with an isolation transformer connected between the offline circuit’s input and
the AC line. Users and constructors of offline circuits must observe this
precaution when connecting test equipment to the circuit to avoid electric
shock.
REPEAT: An isolation transformer must be connected between the circuit input
and the AC line if any test equipment is to be connected.
GND
energy storage at the input utilizes the
high voltage at the input, the load current is limited to the 100mA the buck
converter can supply. If larger transient
loads need to be serviced, the output
capacitor can be sized to support a larger
current for the duration of the transient.
output capacitor until PGOOD went low.
In some cases, using every last joule is
important and the PGOOD pin will remain
high if the output is still within 92% of
the regulation point, even if the input
falls below the lower UVLO threshold
(as might happen if vibrations cease).
A PGOOD output exists that can help
with power management. PGOOD transitions high (referred to VOUT) the first time
the output reaches regulation and stays
high until the output falls to 92% of the
regulation point. PGOOD can be used to
trigger a system load. For example, a current burst could begin when PGOOD goes
high and would continuously deplete the
THE LTC3588-1 EXTENDS
BATTERY LIFE
The battery backup circuit in Figure 9
shows a 9V battery with a series blocking
diode connected to VIN. The piezo charges
VIN through the internal bridge rectifier
and the blocking diode prevents reverse
current from flowing into the battery.
A 9V battery is shown, but any stack of
batteries of a given chemistry can be used
as long as the battery stack voltage does
Though an energy harvesting system can
eliminate the need for batteries, it can also
supplement a battery solution. The system
can be configured such that when ambient
energy is available, the battery is unloaded,
but when the ambient source disappears, the battery engages and serves as
the backup power supply. This approach
300Ω
Figure 12. Solar panel
powering the LTC3588-1
not only improves reliability, but it can
also lead to a more responsive system.
For example, an energy harvesting sensor
node placed on a mobile asset, such as a
tractor trailer, may gather energy when the
trailer is on the road. When the truck is
parked and there is no vibration, a battery
backup still allows polling of the asset.
PZ2
PZ1
IR05H4OCSPTR
+
–
1µF
6V
5V TO 16V
SOLAR PANEL
9V
BATTERY
100µF
25V
4.7µF
6V
PGOOD
VIN
LTC3588-1
PGOOD
10µH
CAP
SW
VIN2
VOUT
+
D0
D1
GND
VOUT
2.5V
10µF
6V
3F
2.7V
NESS SUPER CAPACITOR
ESHSR-0003CO-002R7
April 2010 : LT Journal of Analog Innovation | 7
Not limited to AC sources, DC sources such
as solar panels and thermoelectric couplers
can be used to power the LTC3588-1.
Figure 13. Dual rail power
supply with single piezo
MIDE V25W
PGOOD1
3.6V
PZ1
PZ2
PZ1
PZ2
PGOOD
VIN
VIN
PGOOD
10µH
SW
10µF
6V
LTC3588-1
VOUT
D1
not exceed 18V, the maximum voltage
that can be applied to VIN by an external
low impedance source. When designing a
battery backup system, the piezoelectric
transducer and battery should be chosen
such that the peak piezo voltage exceeds
the battery voltage. This allows the piezo
to “take over” and power the LTC3588-1.
A WEALTH OF ALTERNATIVE
ENERGY SOLUTIONS
The LTC3588-1 can harvest other sources
of energy besides the ambient vibration
energy available from a piezoelectric
element. The integrated bridge rectifier
allows many other AC sources to power the
LTC3588-1. For example, the fluorescent
light energy harvester shown in Figure 10
capacitively harvests the alternating
electric field radiated by an AC powered
fluorescent light tube. Copper panels can
be placed above the light tube on the light
fixture to harness the energy from the
electric field produced by the light tube
and feed that energy to the LTC3588-1
and the integrated bridge rectifier. Such
a harvester can be used throughout
buildings to power HVAC sensor nodes.
D0
1µF
6V
10µF
25V
VIN2
GND
8 | April 2010 : LT Journal of Analog Innovation
CAP
1µF
6V
4.7µF
6V
10µF
25V
CAP
LTC3588-1
D1
D0
Another useful application of the
LTC3588-1 involves powering the IC from
the AC line voltage with current limiting resistors as shown in Figure 11. This
offers a low cost, transformer-free solution for simple plug-in applications.
Appropriate UL guidelines should be
followed when designing circuits connecting directly to the line voltage.
Not limited to AC sources, DC sources
such as solar panels and thermoelectric couplers can be used to power the
LTC3588-1 as shown in Figure 12. Such
sources can connect to one of the PZ1/PZ2
inputs to utilize the reverse current protection that the bridge would provide.
They can also be diode-ORed together
to the VIN pin with external diodes. This
facilitates the use of multiple solar panels
aimed in different directions to catch the
sun at different times during the day.
MULTIPLE OUTPUT RAILS
SHARE A SINGLE PIEZO SOURCE
Many systems require multiple rails to
power different components. A microprocessor may use 1.8V but a wireless transmitter may need 3.6V. Two
LTC3588-1 devices can be connected to
10µH
SW
VOUT
VIN2
4.7µF
6V
PGOOD2
1.8V
10µF
6V
GND
one piezoelectric element and simultaneously provide power to each output as
shown in Figure 13. This setup features
automatic supply sequencing as the
LTC3588-1 with the lower voltage output (i.e., lower UVLO rising threshold)
comes up first. As the piezo provides
input power both VIN rails initially come
up together, but when one output starts
drawing power, only its corresponding
VIN falls as the bridges of each LTC3588-1
provide isolation. Input piezo energy
is then directed to this lower voltage
capacitor until both VIN rails are again
equal. This configuration is expandable
to multiple LTC3588 devices powered
by a single piezo as long as the piezo
can support the sum total of the quiescent currents from each LTC3588-1.
CONCLUSION
The LTC3588-1 provides a unique power
solution for emerging wireless sensor
technologies. With extremely low quiescent current and an efficient energy
harvesting solution it makes distributed
sensor networks easier to deploy. Sensors
can now be used in remote locations
without worrying about battery life. n